Belt tensioners use a system or mechanism to dampen tensioner movement which minimizes steady state vibrations or transient events that cause belt slip. The required magnitude of this damping depends on many drive factors including geometry, accessory loads, accessory inertia, engine duty cycle and others. For instance, drive systems that have higher torsional input or certain transient dynamic conditions may require higher damping to sufficiently control tensioner movement. Although higher damping is very effective at controlling arm movement, it can also be detrimental to other critical tensioner functions (e.g. slow or no response to slack belt conditions). In addition, variation or change in damping that occurs as a result of manufacturing variation, operating temperature and component break-in or wear can also cause the tensioner to be unresponsive.
Damping derived by sliding friction is common among belt tensioners. One particular method involves friction between two surfaces that are perpendicular to the tensioner axis of rotation as in U.S. Pat. No. 4,661,087 to Henderson. The first surface is attached to the tensioner base; the second rotates with the tensioner arm. A spring forces the surfaces together and damping friction is created when the two surfaces slide relative to each other.
Other solutions use wedges that increase damper friction during wind-up, for example U.S. Pat. No. 6,361,459, U.S. Pat. No. 4,878,885, and U.S. Pat. No. 6,863,631, or spring loaded self-energizing brake shoe elements, for example U.S. Pat. No. 6,231,465, U.S. Pat. No. 6,582,332, and U.S. Pat. No. 6,609,988. These designs, however, tend to be complex with many small parts to assemble.
One-way clutch mechanisms have been proposed, for example in U.S. Pat. Nos. 4,583,962 and 6,422,962, for timing belt tensioners for the purpose of preventing or limiting back travel to prevent tooth jump. These “ratcheting” tensioners, however, lack the ability to relieve belt tension sufficiently when not required. Other belt tensioner proposals include, for example U.S. Pat. No. 5,692,482, U.S. Pat. No. 4,832,665, and U.S. Pat. No. 6,375,588, use of a one-way device coupled to a viscous damper. Although these devices offer good functionality, retention of the viscous fluid throughout the service life can be difficult. Yet another design disclosed in U.S. Patent App. Publication 2003/0008739 that uses friction generated by the clamping action of a wrap spring clutch to provide damping.
Most friction dampers provide equal resistance to motion regardless of the direction of tensioner movement. However, tensioner damping that is unequal, or asymmetric, has been shown to provide superior control of tensioner arm movement compared to typical symmetric systems. An asymmetrically damped tensioner provides damping when additional belt tension is encountered but is free to respond to slack belt conditions. Although asymmetric functionality may not be required for all other front end accessory drive tensioners, the potential for increased service life, solving other transient dynamic system problems including belt slip, or simply making the tensioner less sensitive to damping variation makes it a desirable design option.
The aforementioned damping mechanisms and tensioner designs are not ideal. Accordingly, a new damping mechanism and tensioner design is desired.